1. Field of the Invention
The present invention relates to a finite-distance zoom lens having a fixed object-to-image distance which is capable of attaining a zoom ratio of as high as 4 in the low-magnification range alone (or in the high-magnification range if the object and the image are reversed in position).
2. Background Art
A finite-distance zoom lens can be used either in the low-magnification range or in the high-magnification range. The use in a facsimile transmitter is a typical example of the low-magnification application and the use in an enlarger is an example of the high-magnification type of use. Either type of zoom lens is capable of continuous variation of the magnification with the object-to-image distance being held constant.
The known zoom lenses are classified into the following three types according to the range of attainable magnifications.
(I) A zoom lens for use with a still camera which is capable of varying the magnification from zero (at infinite distance) to about one-tenth;
(II) a zoom lens for use with a copier which has a magnification in the neighborhood of unity; and
(III) a zoom lens having an intermediate range of magnification.
a zoom lens of the first type that is capable of attaining a zoom ratio in the range of 4 to 5 is known but it experiences a fairly large distortion (3-5%) and is not suitable for use as a finite-distance zoom lens of the type contemplated by the present invention.
A zoom lens of the second type that is capable of attaining a high zoom ratio in the range of 4 to 9 and which nontheless experiences small distortion is available. The relative lens layout of this zoom lens is substantially the same in both the low- and high-magnification ranges with respect to unity magnification. Its lens configuration is virtually symmetrical enough to allow the distortion to be decreased fairly easily. However, at a magnification in the neighborhood of unity, the distance between the lens and the image plane is so great as to render unattainable a lens system of the type contemplated by the present invention which has a high zoom ratio only in the low-magnification range (or alternatively in the high-magnification range).
Most of the third type of finite-distance zoom lenses having an intermediate range of magnification that have been proposed to data are only capable of attaining a zoom ratio of less than 2. Two exceptions which are capable of a high zoom ratio of 4 are described in Unexamined Published Japanese Patent Application Nos. 214009/1984 and 150020/1985. However, the zoom lens shown in Unexamined Published Japanese Patent Application No. 214009/1984 is complicated in construction in that all of the lens groups are displaced by very large amounts during zooming. Thus, in this respect, there is room for improvement. The system proposed in Unexamined Published Japanese Patent Application No. 150020/1985 requires a smaller amount of displacement of the lens groups and has attained simplicity in the overall construction. However, there still is room for improvement with respect to chromatic aberration at the high-magnification end. In addition, both systems have the disadvantages that they have a large object-to-image distance and that the overall size of the equipment is thereby increased.